U.S. Pat. Nos. 3,091,948; 3,352,127 and 4,116,019 show examples of universal joint bearing seals of the elastomeric, purging type. These seals utilize a single lip for sealing lubricant in the bearing area. When internal lubricant pressures exceeds the design sealing pressure, then the lip will be forced open and lubricant will exude from the bearing area. These seals have certain deficiencies. For example, the purging seal has a static design purge pressure which exceeds the operating pressure due to centrifugal head, however, the seal lip must then operate at high contact pressures at lower speeds, causing lip wear. This wear will reduce the purge pressure below the centrifugal head after a period of operation and the seal lip will leak lubricant when the cross and bearing is operated at higher speeds. The single lip type is un-protected from external contaminants and abrasion which is a secondary cause of lip wear.
U.S. Pat. Nos. 3,138,942; 3,200,615; 3,266,269; 3,377,820 and 3,479,840 show examples of elastomeric dual lip purging seals. The outer lip's principal function is to prevent abrasive contaminants from reaching the primary sealing lip. This type of design minimizes wear due to external contaminants, however, the primary seal lip is identical in design philosophy to a single lip purging seal and is subject to the same wear.
Thus it can be seen that the typical elastomeric purging type universal joint trunnion seal, both single and dual lip, leaks lubricant because of wear.
U.S. Pat. No. 2,996,901 shows an example of a dual reversed lip non-purging seal. The inner lip is the principal seal and the outer lip is for exclusion of external contaminant. This patent also describes an additional separate sealing device for further protection of the primary seal from external contamination. The non-purging seal has deficiencies similar to the purging type. For example, the non-purging seal has a low static design pressure, however, the lip must operate at maximum contact pressure when it is subjected to the centrifugal head at high speed, causing heavy lip wear. This wear has initially little effect on the lip's ability to seal at high speeds, but the wear impairs its ability to seal at low speeds and under static conditions. Eventually the wear becomes excessive and the seal will leak at high speed. Thus it can be seen that an elastomeric non-purging type universal joint trunnion seal leaks lubricant because of wear.
Universal joint cross trunnions and bearings use trunnion seals which fulfill a dual function; they exclude external contaminants and prevent lubricant leakage from the bearing area. Trunnion seals for heavy duty universal joints can be divided in two distinct functional categories, purging and non-purging type seals, both of which utilize dual sealing lips. The outer lip on both type functions as a one-way valve to prevent contaminant flow into the bearing area. The inner sealing lip of a purging seal functions as a one-way valve to allow lubricating fluid to flow out of the bearing at a pressure exceeding the maximum operating pressure due to centrifugal head. Conversely the non-purging seal has a reversed inner lip which acts as a one-way valve to prevent lubrication fluid flow at any pressure, other than vacuum.
The purpose of a purging trunnion seal is to allow lubrication fluid flow through a central lubrication fitting to all four bearings and to provide a means for purging out old lubricant or external contamination which penetrated the outer lip.
A reverse lip non-purging seal tightens against the trunnion as internal pressure increases, thus it is incapable of lubrication through a central lubricating fitting unless a method for pressure relief is provided. If a relief system is not available, the assembly must be dismantled and hand-packed with lubricant. Crosses are drilled through the four trunnions to provide holes which intersect at the center of the cross body. These holes have a dual purpose; one is to provide passages for lubricating all four trunnions through a central lubrication fitting and the other is to provide a reservoir of lubricant to feed upward into the bearing area to replace lubricant lost through the sealing lip. The column of lubricant in the cross lubricant hole of an 8.50" swing diameter universal joint exerts a centrifugal head of approximately 25/30 psi for example, at the seal lip when the cross and bearing assembly is rotating at 3,000 RPM.
An 8.50" swing diameter joint purging seal has a design purge pressure of approximately 50 psi which exceeds the operating pressure due to centrifugal head, however, the inner seal lip must then operate at high contact pressures at lower speeds, causing inner lip wear. This wear will reduce the purge pressure below 25/30 psi after a period of operation and the seal lip will leak lubricant when the cross and bearing is operated at higher speeds.
A non-purging seal inner lip is subject to maximum lip contact pressure and wear when it is operated at high speed. Although this wear has little initial effect on its ability to seal at high speeds, the wear does impair its ability to seal at low speeds and under static conditions. Eventually, excessive wear causes the seal to leak at high speed. Both purging and non-purging seals are forced to operate at lip contact pressures which are considered to be excessive for elastomeric type seals and which causes them to wear and ultimately leak. Because of this wear, grease is used as a lubricant to minimize the leakage problem, although it is well known that a 90 or 120 weight oil is a superior lubricant for universal joint cross and bearings.
Cross and bearing operating pressure is also a function of operating temperature because the assembly is essentially completely filled with lubricant at ambient temperature and there is no provision for expansion.
Internal pressure in a cross and bearing with purging type seals will increase as the operating temperature rises until it reaches the purge pressure of the inner lip (50 psi, for example). At this point, lubricant will be expelled and the pressure will be relieved. Tests indicate that more lubricant fluid volume is expelled than is necessary to simply relieve pressure and air is ingressed through the temporarily open lip. This assumption is based on the fact that internal pressure does not go to vacuum when the cross trunnion and bearing is cooled to room temperature, which would be the case if only the precise amount of grease was expelled. Repeat heating of the cross and bearing may expel additional fluid, however, at some point in the heat/cool cycle the volume of internal air becomes large enough so that the pressure caused by expansion of the fluid/air mixture is less than the purge pressure of the inner lip and no further lubricant leakage occurs.
Since a reversed lip non-purging seal is not capable of pressure relief, it can be seen that the cross trunnion and bearing internal pressure is a function of lubricant temperature, coefficient of expansion, and bulk modulus. In a typical cross trunnion and bearing, this pressure has been measured at 150 psi at 200.degree. F. This amount of internal pressure will result in heavy seal lip wear. When the cross and bearing is cooled to ambient, the internal pressure goes briefly to vacuum which suggests that a non-purging seal will have a tendency to ingress external contaminant. The addition of a low pressure venting system will prevent the seal from operating at high internal pressure due to changes in operating temperatures.